Dye compositions should have optimal stability so that they do not precipitate during transportation or in storage. Typically they should be stable for a prolonged period between 0 and 5° C., but also at around −20° C. and 50° C., respectively. Frozen dye compositions shall be stable after thawing and should not present any stability problems during pumping. Dye compositions containing precipitates can cause disruption in pumping or metering systems and lead to unacceptable machine shutdowns and costly cleaning and maintenance.
One problem of known aqueous dye solutions is the large amounts of added solubilizers, which lead to a high carbon content level in the dyehouse or paper mill effluents. This leads to effluents of high total organic carbon (TOC) and chemical oxygen demand (COD), and hence causes high water-treating costs.
Concentrated aqueous dye solutions are known. For example, EP0369940A2 discloses aqueous dye solutions comprising 7% to 30% by weight of a benzothiazole dye derivative together with 1 to 5 mol of a specific amine per mole of dye and also 10% to 25% by weight of an organic solubilizer.
WO03064539A1 discloses aqueous dye solutions comprising 5% to 30% by weight of a dye based on a benzothiazole derivative together with 0.05-5% of one or more additional, aromatic heterocycles (for example an additional benzothiazole derivative) and 1 to 5 mol of a base or of a mixture of bases per mole of dye. To produce the stable dye solutions, in both cases the free dye acids are isolated and stirred up with mixtures of water, standardizers and solubilizers.
Further reference can be made to EP 0 167 952, which discloses a number of concentrated aqueous solutions of dyes groups rendering those dyes water-soluble.